440 FXUS66 KPQR 240350 AFDPQR Area Forecast Discussion National Weather Service Portland OR 843 PM PDT Mon May 23 2016 .SYNOPSIS...A broad area of low pres over the region will gradually drop further southward, resulting in drier weather for Tuesday and Wednesday with near normal temperatures. This break will be short lived as another upper low pressure will arrive to close the week. This will bring showers and cooler temperatures to the region for the coming holiday weekend. && .SHORT TERM...(Tonight through Thursday)...A broad upper level trough apparent on water vapor imagery will more or less continue to linger across the Pacific Northwest and western US for the next several days. With that said, the atmosphere will gradually warm and dry out aloft Tuesday and Wednesday as weak shortwave ridging tries to temporarily nose into the region from the west. This should bring considerably fewer showers to the area when compared to the past several days. In the immediate short term...visible satellite imagery and doppler radar indicate plenty of showers developed this afternoon and early this evening. Showers have generally been fairly weak with limited instability and equilibrium levels generally remaining under 15kft. An area of weaker flow, in part due to a weak low pressure spinning southward offshore, did allow a few slow moving showers to develop over the north Oregon coast range, but even here rainfall amounts have generally remained under a quarter of an inch. With the loss of daytime heating, expect showers to decrease overnight with minimal rain chances by morning. As 500mb heights build overhead tonight and Tuesday the lower to middle part of the atmosphere overhead will warm and dry out when compared to today. This should produce considerably fewer showers Tuesday. Nonetheless, there may be just enough instability present during the afternoon to allow a few showers to develop over the central Oregon coast range and particularly the Oregon Cascades. There is a slight chance a shower or two will drift into the central and southern Willamette Valley during the late afternoon, but most folks should remain dry. Models are noticeably drier and more stable farther north towards PDX so PoPs were nudged below mentionable criteria. Models in varying shapes and forms, hint that a weak low level front/trough will push south-southeastward towards the Pacific Northwest late Tuesday night and Wednesday morning. This front appears to be the last gasps of low level feature associated with the shortwave trough apparent on water vapor imagery currently diving south-southeastward across the Alaska Panhandle. Model soundings and QPF fields indicate enough low level moisture and instability will move into the region that a few showers seem plausible along the coast and coast range early Wednesday. Given the shallow nature of this moisture instrusion, generally below 850mb, not sure how much the chance for wet weather will extend inland. Nonetheless, a few light showers may develop across the interior in the late morning/early afternoon as the lower atmosphere turns over, particularly across the north. PoPs were trended towards this general evolution in mind. Models are in modest agreement that a shortwave trough currently over western Alaska will slide south-southeastward and approach the region Thursday. This should reinforce marine clouds and keep temperatures cool. It may increase light shower chances, but model soundings are not terribly impressive so at this point, it appears any showers that develop would be very light. And there is a decent chance showers associated with this system may very well hold off until Thursday night...or Friday for that matter. /Neuman .LONG TERM...Thursday night through Monday. Previous discussion below...Forecast models are in reasonable agreement for the first part of the weekend in terms of the overall pattern. An upper low dropping into southern B.C. late Thursday will slide south across Washington and Oregon on Friday. This will support another round of widespread showers across the region and afternoon temperatures will once again be a few degrees below seasonal norms. Some slight differences in how the models evolve this upper low, but the general trend in the latest GFS and ECMWF reflect that the low will push south and east and allow heights to begin to rise over the region by the end of the weekend. That said, ensemble remembers still have a significant spread of possibilities and the GEM would also keep some showers lingering through the weekend. All of that said, did maintain some slight chance PoPs through the weekend, but tapered these off later Sunday into Monday. If this ridge does begin to push over the Pacific NW by the end of the weekend, expect slightly warmer temperatures and decreasing clouds -though this still remains a lower confidence forecast at this time for the reasons stated above. Snow levels for the weekend may be as low as 5,000 feet late Friday, but increasing to generally above 6,000 feet for the rest of the weekend. /Cullen && .AVIATION...Scattered showers this evening will taper off considerably overnight. VFR conditions currently prevailing across the region, with cigs around 5 to 8 kft. VFR should persist through most of the night. Cigs likely will settle a bit overnight over the interior. There is a chance that cigs may briefly drop to 2500-3000 ft between 12Z and 18Z Tue, but think it is more likely that VFR conditions persist over the interior. The fcst models indicate that MVFR or IFR stratus will reform along the coast later tonight...likely 09Z-12Z as northerly flow becomes established. Any lower clouds that manage to form Tue AM should lift to VFR by midday, with widespread VFR expected Tue afternoon and evening. The mountains will remain mostly obscured into Tue AM. PDX AND APPROACHES...Expect mainly VFR conditions to prevail through Tue evening. There is a chance that cigs will lower to around 2500-3000 ft between 12Z and 18Z Tue, but think it is more likely that conditions remain VFR. Pyle && .MARINE...High pressure building over the waters this evening, turning winds northerly overnight into Tuesday morning. High pressure remains over the waters through much of the week with northerly winds. Will see winds increase to near 20 knots each afternoon/evening starting Tuesday through the end of the week, with weaker winds expected each morning. As of now, not seeing winds get much above 20 knots. Gradients don't appear to be strong enough until possibly Sunday as an upper-level low drops south out of British Columbia. Seas are currently around 6 to 7 ft, with dominant wave periods around 9 seconds. With persistent northerly winds, we will see fresh swell start to build into the waters from the north- northwest, which could bring hazardously steep seas to the outer waters beyond 10 nm possibly as soon as Tuesday evening, but more likely Wednesday afternoon and evening. Seas will build to around 8 ft with dominant wave periods dropping to around 8 seconds by Wednesday evening. Outer waters beyond 10 nm from the Coast will probably see steep seas around 7 to 8 ft with wave periods around 8 seconds through Friday, though there is a chance that seas will be less steep around 6 ft during morning hours when winds will be weaker. Over the weekend, the upper-level low may disrupt the northerly wind pattern to our north, which could allow seas to become less steep. Right now, though, much uncertainty exists with the track of this system. Pyle/LM && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...None. && $$ Interact with us via social media: www.facebook.com/NWSPortland www.twitter.com/NWSPortland This discussion is for Northwest Oregon and Southwest Washington from the Cascade crest to 60 nautical miles offshore. The area is commonly referred to as the forecast area.
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